interp: fix method lookup for aliased types

Fixes #1058.

Author: mvertes

interp/cfg.go

@@ -1433,7 +1433,7 @@ func (interp *Interpreter) cfg(root *node, importPath string) ([]*node, error) {
 				// TODO(mpl): move any of that code to typecheck?
 				c.typ.node = c
 				if !c.typ.assignableTo(typ) {
-					err = fmt.Errorf("cannot use %v (type %v) as type %v in return argument", c.ident, c.typ.cat, typ.cat)
+					err = c.cfgErrorf("cannot use %v (type %v) as type %v in return argument", c.ident, c.typ.cat, typ.cat)
 					return
 				}
 				if c.typ.cat == nilT {

interp/type.go

@@ -1065,6 +1065,10 @@ func (t *itype) methods() methodSet {
 		seen[typ] = true
 
 		switch typ.cat {
+		case aliasT:
+			for k, v := range getMethods(typ.val) {
+				res[k] = v
+			}
 		case interfaceT:
 			// Get methods from recursive analysis of interface fields.
 			for _, f := range typ.field {